Right now, not much of anything at all. It boots from a 1.44mB floppy disk, and enters into a BASIC interpreter, just like your favorite home computers of the 70s/80s!
Currently the BASIC only understands simple, 1-digit arithmetic expressions. But this will soon change; I intend to implement at least as many features as uBASIC, maybe QuickBASIC eventually.
Keep in mind that the unit tests are compiled with gcc, not bcc, as I couldn't find an elegant way to make bcc produce objects that could easily be run in linux (especially Travis-CI). So while the tests are an accurate test of the C code under ANSI spec, they make no promises re: what bcc is eventually going to do with that code.
New unit tests are created by making a new *.c file in tests/. Drop the name of whatever portions of piquant it relies on into a '.deps' file. Look at the others for examples.